1. Field of the Invention
This invention relates to a sound attenuating muffler, and more particularly to a muffler for damping sound waves of specific frequencies.
2. Discussion
Mufflers are generally incorporated in automobile exhaust systems to limit the sound pressure level of exhaust noise produced by engine operation. There are two general classifications of mufflers, reactive and dissipative. Reactive mufflers are generally composed of a number of resonating chambers of different volumes and shapes connected with pipes. Reactive mufflers may include baffles or flow reversals. However, these configurations produce a relatively high pressure drop, causing a backpressure at the exhaust of the engine, thus restricting engine performance. Dissipative mufflers are usually composed of ducts or chambers which are filled with acoustic absorbing materials such as fiberglass, steel wool, or a porous ceramic. These materials absorb the acoustic energy and transform it into thermal energy. Unfortunately, the sound absorbing material in dissipative mufflers tend to break down because of the velocity of the material and the high velocity and temperature of the exhaust. Mufflers consisting of a combination of the reactive and dissipative types are known in the art in a variety of configurations.
The prior art muffler systems generally fail to attenuate sound waves over a broad band of frequencies. Mufflers typically provide effective attenuation only at specified frequencies equal to or greater than a specific cut-off frequency. The transmission loss, or effectiveness under ideal conditions, of a typical dissipative muffler is generally an inclined straight line with respect to frequency, and provides effective attenuation only above approximately 500 Hertz. As a result, the typical dissipative muffler fails to attenuate low frequency sound. This failure is unacceptable in an automobile exhaust muffler because the sound produced by the engine has greatest amplitude at lower frequencies, such as below approximately 500 Hertz. The transmission loss of a typical reactive muffler or expansion can is generally a periodic series of sinusoidal "humps." As a result, a reactive muffler provides acceptable amplitude levels of low frequency attenuation, but exhibits a series of "zero frequencies" where the muffler provides no attenuation. It is desirable to combine the accoustic performance of both types of mufflers to achieve broad band low frequency attenuation in a low back pressure muffler.